In recent years, there has been increasing interest in non-oxide ceramics, such as metal nitrides and carbides, that possess high temperature strength and corrosion resistance. Among these materials, aluminum nitride (AIN) is especially important because of its unique physical properties. For example, AIN has a thermal conductivity close to that of metals and more than 10 times that of alumina (Al.sub.2 O.sub.3), a coefficient of thermal expansion comparable to silicon and silicon carbide, a high electrical resistivity, and mechanical strength comparable to alumina ceramics.
Metal nitride powders can be made in various ways. For example, a metal oxide powder, such as Al.sub.2 O.sub.3, zirconia (ZrO.sub.2), or titania (TiO.sub.2), can be mixed with an excess of a carbonaceous powder and heated to a temperature above 1100.degree. C. in a nitrogen-containing atmosphere. The metal nitride powder formed by this method is, however, mixed with unreacted carbonaceous powder that detracts from the properties of the metal nitride powder. The unreacted carbonaceous powder can be removed by oxidizing it at temperatures between about 600.degree. C. and about 700.degree. C. At these temperatures, however, a portion of the metal nitride powder also can oxidize.
U.S. Pat. 4,975,260 to Imai et al. teaches an alternate method for making a metal nitride powder by reacting a metal oxide or metal hydroxide powder with a gaseous mixture of ammonia (NH.sub.3) and a hydrocarbon at a temperature ranging from 1300.degree. C. to 1600.degree. C. Although this method is an improvement over some prior art methods, it still leaves residual carbon in the metal nitride product. Moreover, it requires a temperature of at least 1300.degree. C. As with any synthesis process, it is desirable to keep energy consumption as low as possible.
Therefore, what is needed in the industry is a method of making metal nitride powder at a relatively low temperature.